A burner that supplies high temperature gases to a process or an external combustion engine should have high thermal efficiency, low emissions, good cold starting capabilities and a large turndown ratio or wide dynamic range. High thermal efficiency may be achieved by capturing the thermal power in the hot exhaust exiting the load-heat-exchanger. For example, in a Stirling engine, the exhaust gas exits the load heat exchanger or heater head at about 900° C. Typically, this thermal power is captured by preheating the incoming combustion air in a recuperative or regenerative heat exchanger. The preheated air typically enters the fuel mixing section at 500 to 800° C. Low emissions in burners are best achieved by vaporizing and mixing the fuel with the air before the mixture reaches the burner's combustion zone. In addition to producing high efficiency and low emissions with preheated air, the burner must be capable of being ignited and warmed-up with ambient temperature air. It is desirable that the burner be capable of good fuel/air mixing, quickly reach full power, and produce a stable flame over a wide range of air temperatures and fuel flows.
Supplying gaseous fuel to high efficiency burners presents a number of challenges. The major challenge is getting gaseous fuel supplied at low pressure into the high efficiency burners that typically operate at elevated pressures. Most of the common gaseous fuels such as propane, natural gas and biogas are generally supplied at low pressure, typically 3 to 13 inch of water column (in.w.c). The high efficiency burners operate at elevated pressures to overcome the pressure drops associated with the recuperative heat exchangers, the load heat exchanger and the mixing requirements of the burner. Typically, the air pressure upstream of the combustion chamber operates at pressures from 5 to 25 in.w.c. Existing gaseous fuel burners address these challenges by using gaseous fuel pumps possibly in combination with throttle devices. Gaseous fuel pumps are not commercially available below 100 kW thermal. If such pumps were built, they would also have to be approved by at least one listing agency. Furthermore, fuel pumps and throttles require additional power and controls all of which increase the cost of the final device, reduce the net power of engines and increase the energy costs to the burner systems. There is therefore the need for simple, efficient, and affordable solutions to delivering low-pressure gaseous fuel to high efficiency burners.
High efficiency recuperative burners are an important component to external combustion engines and other processes requiring heat at high temperatures. External combustion engine include steam engines and stirling engines. Thermal-Photovoltaic generators are an example of a non-engine high temperature load that would benefit from a high efficiency recuperative burner.
External combustion engines, for example Stirling cycle engines, have a long technical heritage. Walker, Stirling Engines, Oxford University Press (1980), describing Stirling cycle engines in detail, is incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. Other external combustion engines are steam engines, organic Rankine engines and closed cycle gas turbine engines.